1. Field of the Invention
The present invention relates generally to an injection molding apparatus, and in particular to an injection molding apparatus having a nozzle with a controlled heat distribution along the nozzle channel.
2. Background of the Invention
Hot runner nozzles often have an uneven distribution of heat along the length of the melt channel when operating in an injection molding apparatus. The nozzles are generally housed in an opening in a mold plate between a manifold and a mold cavity. The mold plate is typically cold, particularly near the manifold and the mold cavity. As a result, a nozzle head portion, and the area around a nozzle tip, tend to lose more heat through direct contact with the mold than the nozzle mid-section, which does not contact any part of the mold plate. The uneven distribution of heat along the nozzle causes the temperature of the melt flowing through the nozzle to vary as it travels toward the mold cavity. Any variation in melt temperature can adversely affect the quality of the molded products and is therefore undesirable.
A further disadvantage of an uneven temperature distribution along the length of the nozzle is that the nozzle is subjected to high stress due to the continuous cycling between higher and lower temperatures. This can result in a shorter nozzle life.
With the increased use of plastic materials that are more sensitive to fluctuations in temperature, the melt must be maintained within more accurate and controllable temperature ranges. If the temperature rises too high, to compensate for the heat loss through the contact with the mold, degradation of the melt will result; and if the temperature drops too low, the melt will clog in the system and produce an unacceptable product. Both extremes can necessitate the injection molding apparatus being shut down for a clean-out, which can be a very costly procedure due to the loss of production time.
There is therefore a need to provide a hot runner nozzle having a generally uniform temperature distribution along the length thereof.